ULTRASONICS: Cleaning Silently with Sound Waves

T.K. Hareendran is founder and promoter of TechNode Protolabz

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Often, we struggle to clean electronic gadgets and other gizmos, especially those having small, delicate, inaccessible parts. The proven solution to such a need is an ultrasonic cleaner. In order to understand how an ultrasonic cleaner works, we should look at the techniques behind it and its inner electronics.

Nothing but a bubble

At the heart of ultrasonic cleaning is a bubble. Bubbles are created by ultrasonic sound waves (having frequencies higher than audible sound) as these move through water. This is known as cavitation, which is simply the formation of bubbles (cavity) in water. Since bubbles created through cavitation are just empty spaces, there is nothing to keep these open. As a result, bubbles implode almost as fast as these are created.

In an ultrasonic cleaner, this happens millions of times per second. This constant implosion produces tremendous vacuum energy in the form of heat and pressure that gives enormous cleaning power to ultrasonics. When cavitation happens near a dirty object, vacuum action produced by those millions of bubbles constantly imploding creates a tiny pressure wave that reaches deep into every nook and cranny of even the most delicate items. This tiny pressure wave dislodges and breaks up the dirt and other contaminants and gently lifts it away. The result is very fast and effective cleaning.

Portable ultrasonic cleaner
Fig. 1: Portable ultrasonic cleaner

The silent scrub

You can use ultrasonic cleaning for fast and perfect removal of contaminants from objects by immersing these in a tank of fluid flooded with ultrasonic waves. Ultrasonic energy enters the fluid within the tank and causes the rapid formation and implosion of minute bubbles. The bubbles rapidly increase in size until these implode against the surface of the object immersed in the tank. The resulting enormous energy release lifts contamination off the surface and innermost recesses of even intricately-shaped parts.

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Ultrasonic cleaning is the ultimate solution for cleaning contact lenses, jewellery and metal goods such as dental tools, watches and screws. With the right method, ultrasonic cleaning can thoroughly strip the object of dirt, dust, grime and other contaminants, both on the outer and inner surfaces, without causing any damage to the object.

The cleaning process is non-destructive and produces consistent cleaning results of the highest standard, time after time. Ultrasonic cleaning is an energy-efficient, environmentally-friendly and technically-feasible alternative to cleaning solvents that are ozone-depleting and have other undesirable characteristics. Reduced emissions, biodegradable waste and a safer working environment are among the many benefits of using an ultrasonic cleaner.

Process of ultrasonic cleaning
Fig. 2: Process of ultrasonic cleaning

Cleaning machines

Nowadays, in addition to industry-grade equipment, lightweight (portable/desktop) ultrasonic cleaners are also available for day-to-day cleaning of objects. These cleaners come in a range of sizes that have different working frequencies. Desktop models with lower frequency (40kHz -70kHz) waves are good for home and dental equipment. Powerful cleaners capable of making ultrasonic frequencies as high as 200kHz (70kHz -200kHz) are expensive, but worth the investment for precious jewellery and objects with fragile parts, such as watches and contact lenses.

Types of ultrasonic cleaners
Fig. 3: Types of ultrasonic cleaners

Basic components of an ultrasonic cleaner include an ultrasonic transducer, an ultrasonic generator and a tank filled with aqueous solution. The key component is the transducer that generates the high-frequency mechanical energy. Outer body of the ultrasonic cleaner consists of a stainless steel tank (usually accompanied by a steel mesh basket to keep the object), also known as the ultrasonic tank.

A transducer is usually placed at the bottom or on the sides, or sometimes both when watt density is a concern. It can be welded directly into the tank, or a watertight immersible unit can be placed directly into the aqueous solution. The ultrasonic generator converts a standard electrical frequency of 50Hz – 60Hz into high frequency required for the task, in the ultrasonic range.

A piezoelectric transducer, commonly used in common models, contains piezo-ceramic crystals that change their size and shape when exposed to ultrasonic waves. This causes the sides and bottom of the tank to vibrate and form innumerable microscopic bubbles in the fluid. As these expand and compress alternately, a vacuum cavity builds up within these.

In addition, the crystals’ constant impact against each other produces heat energy. Since cleaning of some objects calls for precise time and temperature control, a timer and heater circuitry is also included in the ultrasonic cleaner. (Heated ultrasonic cleaning improves the cleaning efficiency by removing trapped air in the fluid.)

Besides, auto-follow electronic circuitry in latest ultrasonic generators is designed to maintain the centre frequency when the ultrasonic tank is subjected to varying load conditions. When parts to be cleaned are placed in the tank, or when water level changes, the load on the generator changes. With auto-follow circuitry, the generator matches electrically with the mechanical load, providing optimum output at all times to the ultrasonic tank.

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